@VINT64 Honestly I get that she does need more room both to move around in and to grow, but releasing her into the wild at this point is probably one of the most wildly irresponsible things he has tried to do yet.
Even if he did release her into the wild, who is going to teach her how to hunt?
She probably will never see her actual parents again, and being an adolescent with mostly human physiology she can't reasonably defend herself from predators in the ocean all by herself. This is like saving a baby bird that's fallen out of it's nest only to release it again before it's wings have even fully developed. At this point she would likely be eaten alive in the depths if he left her. Try dumping a 9 year old human out into the wilds alone and see how well they do, she isn't an animal, so she probably isn't going to suddenly mature within 1 to 2 years like a rescued fox or deer would. Him even trying this at this point was dumb, the ocean is not a forgiving place, it's what he rescued her from to begin with.